Bauxite as mined generally contains from about 0.1 to 0.3 wt. % of organic compounds expressed as organic carbon, but occasionally organic carbon contents of up to about 0.6% are found. It is generally believed that the organic compounds are present in the form of humic substances. Typically, on digestion of bauxite in the Bayer process, about half of these organic compounds are extracted into the liquor. With recycling of the Bayer liquor, the concentration of these humic substances and their degradation products build up to an equilibrium concentration which depends upon the amount of organic compounds present in the bauxite and organic losses during processing.
There are other possible sources of organic compounds in Bayer liquor. These sources include organic compounds added as flocculating and anti-foaming agents. However, such compounds represent only a small portion of the total organic carbon content in recycled Bayer liquor.
The accumulated humic matter and its breakdown products together with other organic compounds in Bayer liquor are known to cause numerous process problems. Difficulties caused by these organic impurities in Bayer liquor are, for example, low alumina yield, production of excessive fines, poor red mud settling, high calcium and sodium oxide levels in calcined alumina, caustic loss due to the formation of sodium organic compounds, colored aluminum trihydroxide, increased liquor density, viscosity and boiling point, and foaming of the liquor.
Numerous methods are known in the prior art for controlling organic carbon impurities in Bayer process liquor. These methods include treatment of the process liquor with sodium hypochlorite or other oxidizing agents such as oxygen or air (U.S. Pat. No. 4,335,082). Another process, described in German Patent Off. No. 2,415,872 involves the addition of a calcium compound to the process liquor to remove humic matter as insoluble calcium compounds. U.S. Pat. No. 4,046,855 describes removal of humic compounds by addition of a magnesium compound to the liquor. U.S. Pat. No. 4,101,629 discloses purification of Bayer liquor by addition of a barium compound. Still more methods can be found in the literature which involve removal of oxalates, desalting processes, and using an ion exchanger. All of these methods involve removal of organic compounds from process liquor. A method in which organic compounds are removed from bauxite itself involves roasting the bauxite at an elevated temperature. However, this process requires a high energy consumption, and it lowers the recovery of alumina, thereby making it less attractive from a cost standpoint.
It is a principal object of the present invention to provide a process for removing substantial amounts of organic matter from an aluminum-containing ore such as bauxite without reducing substantially the alumina content of the ore.
A related object of the invention is to provide an efficient and economical process for removing organic contaminants from mineral ore.
Another object of the invention is to provide a beneficial use for plant lake water formed in the Bayer process.
Additional objects and advantages of the invention will be apparent to persons skilled in the art from the following specification.